Use ECMGenerateExportHeader to manage deprecated API better

Summary:
Allows

• projects linking to KCalendarCore to hide deprecated API up to a given version or silence deprecation warnings after a given version, using
  • -DKCALENDARCORE_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DKCALENDARCORE_NO_DEPRECATED
  • -DKCALENDARCORE_DEPRECATED_WARNINGS_SINCE
  • -DKCALENDARCORE_NO_DEPRECATED_WARNINGS
    
    or
  • -DKF_DISABLE_DEPRECATED_BEFORE_AND_AT
  • -DKF_NO_DEPRECATED
  • -DKF_DEPRECATED_WARNINGS_SINCE
  • -DKF_NO_DEPRECATED_WARNINGS
• to build KCalendarCore optionally with deprecated API excluded from the build, using "EXCLUDE_DEPRECATED_BEFORE_AND_AT" cmake argument.

Test Plan: Builds with EXCLUDE_DEPRECATED_BEFORE_AND_AT set to 0, 5.64.0, CURRENT.

Reviewers: Frameworks, vkrause

Reviewed By: vkrause

Differential Revision: https://phabricator.kde.org/D24682